This invention relates to a power steering (or assisting) device of an energy saving type, by which steering assistance can be changed in response to changes in operating speed of an engine or to changes in vehicle speed.
In general, it is well known that, preferably, in a power steering device, a supply of fluid under pressure to a power steering motor (actuator) is increased to increase steering assistance when a vehicle is stationary or moving at a slow speed. The supply of fluid under pressure conducted to the power steering motor is decreased to reduce the steering assistance at higher vehicle speeds to thereby increase stability during high speed steering maneuvers.
Heretofore, in order to obtain the above characteristic in a power steering device it has been suggested to provide a power steering device as disclosed in Japanese Patent Publication No. 54-9774. This device uses an electromagnetic clutch to drive a pump which discharges the power steering fluid. The electromagnetic clutch is operated to assist steering when the angle of rotation of the steering wheel exceeds a predetermined value. A larger angle of rotation of the steering wheel usually is required when the vehicle is stationary or moving at a low speed. However, in such a known device, since the degree of power steering is determined by the angle of rotation of the steering wheel, the steering assistance is suddenly applied in the proximity of the predetermined angle of rotation, this is an unstable state which may invite a dangerous condition.
Further, as disclosed in Japanese Patent Publication No. 54-11170, it has been suggested to provide a power steering device in which the supply of the power fluid from an oil pump to a power cylinder is controlled by an electromagnetic bypass valve or is adjusted by a flow adjusting valve in response to a running state of a vehicle. However, the power steering device of this type spends much energy and cannot meet the needs of the energy saving times because, at a higher speed running state of the vehicle, most of the power fluid discharged from an oil pump, which rotates continuously at higher speed in proportion to rotating speed of an engine, is returned to an oil tank without working.
Recently, a measure to save energy shows a tendency to decrease the speed of rotation of an engine at an idling state and to raise the maximum rotational speed of the engine so as to miniaturize and lighten the weight of the engine. However, in a conventional oil pump discharging power fluid and being driven by the engine, there are disadvantages in that the engine will drive the pump at a speed which is insufficient to enable the pump to supply the required power steering fluid when the vehicle is stationary or moving at a slow speed.